1. Field of the Invention
The present invention relates generally to the system configuration and methods for controlling and operating a projection apparatus. More particularly, this invention related to an image projection apparatus implemented with a plurality of spatial light modulators and light sources with a controller to control the modulators in different modulations states in coordination with the light sources emitting pulsed emissions to achieve optimal quality of image display.
2. Description of the Related Art
After the dominance of CRT technology in the display industry for over 100 years, Flat Panel Display (hereafter FPD) and Projection Display became popular because of its smaller form-factor and larger size of the screen. Among several types of projection displays, projection displays using micro-display are gaining recognition by consumers because of higher picture quality as well as lower cost than FPDs. There are two types of micro-displays used for projection displays in the market: micro-LCD (Liquid Crystal Display) and micro-mirror technology. Because a micro-mirror device uses the randomly polarized light, it is brighter than a micro-LCD, which uses polarized light.
Even though there have been significant advances made in recent years on the technologies of implementing electromechanical micro-mirror devices as spatial light modulator, there are still limitations and difficulties when these are employed to display high quality images. Specifically, when the display images are digitally controlled, the image quality is adversely affected because the image is not displayed with a sufficient number of gray scales.
The on-and-off states of micro-mirror control schemes, as that implemented in the U.S. Pat. No. 5,214,420 and by most of the conventional display systems such as that disclosed in U.S. Pat. No. 5,285,407, impose a limitation on the quality of the display. Specifically, with conventional configurations of the control circuit, the gray scale of conventional systems (PWM between ON and OFF states) is limited by the LSB (least significant bit, or the least pulse width). Due to the On-Off states implemented in the conventional systems, there is no way to provide a shorter pulse width than the LSB. The least brightness, which determines the gray scale, is the light reflected during the least pulse width. A limited gray scale leads to lower image quality.
In a simple example, and assuming n bits of gray scales, the frame time is divided into 2n−1 equal time slices. For a 16.7 milliseconds frame period and n-bit intensity values, the time slice is 16.7/(2n−1) milliseconds
Having established these times for each pixel of each frame, pixel intensities are quantified, such that black is 0 time slices, the intensity level represented by the LSB is 1 time slice, and the maximum brightness is 2n−1 time slices. Each pixel's intensity determines it's the length of time the pixel is turned on during a frame period. Thus, during a frame period, each pixel with a value of more than 0 is on for the number of time slices that correspond to its intensity. The viewer's eye integrates the pixel's brightness so that the image appears the same as if it were generated with analog levels of light.
For addressing deformable mirror devices, PWM receives the data formatted into “bit-planes”. Each bit-plane corresponds to a bit weight of the intensity value. Thus, if each pixel's intensity is represented by an n-bit value, each frame of data has n bit-planes. Each bit-plane has a 0 or 1 value for each display element. In the example described above, each bit-plane is separately loaded during a frame, and the display elements are addressed according to their associated bit-plane values. For example, the bit-plane representing the LSBs of each pixel is displayed for 1 time slice.
Projection apparatuses, such as those described above, generally use a light source such as a high-pressure mercury lamp or a xenon lamp. However, these types of light sources perform poorly in high-speed switching that alternate between the ON and OFF states. Because of this, these lamps are commonly controlled to be in a continuous ON state while the apparatus is in operation. Thus, it is not possible to accurately control the light intensity in the transition state, between an ON state and an OFF state, for an ON/OFF modulation of a mirror. This causes a degradation of image quality in the modulation control of a video image when using a spatial light modulator.
Furthermore, when the intensity of light modulated by a spatial light modulator is only controlled by the ON/OFF operation of the mirror, the oscillation speed of the mirror needs to be increased in order to implement a finer control of the light intensity. Increasing the oscillation speed of the mirror, however, is limited by a number of factors including the strength of a hinge constituting the mirror and the frequency of the control signal used for the tilt (i.e., oscillation) control, such as the ON/OFF control. Thus, there will be a limitation in controlling light intensity when only the ON/OFF controls of the mirror are used to control the modulation of light intensities.
In order to control the color temperature and/or color balance, the input video signal needs to be processed. Because of this, further technical problems, such as an unnecessarily complex process circuit for the video, are introduced.